1. Field of the Invention
The present invention relates to a transfer device for transferring onto a recording paper a toner image formed on a photosensitive body in an electrophotographic recording manner and, in particular, to an improvement in the structure of such a transfer device for preventing paper jamming and transfer missing in the transfer process.
2. Description of the Related Art
Such an electrophotographic recorder as a copying machine, a laser beam printer, or so on has generally such an arrangement as shown in FIG. 14, which includes a photosensitive body 1, a developing unit 2, register rollers 3a and 3b, a recording paper 4, a register guide 5, a transfer device 6, a fixing guide 7, the parts 2, 3a, 3b, 4, 5, 6 and 7 being disposed around the photosensitive body 1.
The recorder is arranged so that, as the body turns, the photosensitive body 1 is charged and developed to form a toner image 1a on the body and then the toner image 1a is transferred onto the recording paper 4 through the transfer device 6.
The transfer device used in such a sort of prior art electrophotographic recorder is arranged so that, as shown in FIG. 15, a charging wire 6b is extended within a casing 6a of a trough shape made of an electrically conductive material and along the longitudinal direction of the casing 6a. More specifically, the charging wire 6b is extended as tensioned by holding members 6c of an insulating material mounted at both ends of the casing 6a to have a suitable tension.
Referring again to FIG. 14, the transfer device 6 is disposed so that an opening of the casing 6a is opposed to the periphery of the photosensitive body 1. The casing 6a is grounded while the charging wire 6b is connected to a voltage application circuit.
In the illustrated example, the casing 6a and photosensitive body 1 have a length more than the maximum recording length of the electrophotographic recorder.
In a recording mode, the recording paper 4 is fed into a gap between the photosensitive body 1 and transfer device 6 through the register rollers 3a and 3b and through the register guide 5, while, the toner image 1a formed on the peripheral surface of the photosensitive body 1 is shifted as the body 1 rotates, to a position where the toner image 1a faces the transfer device 6.
Applied at this position to the charging wire 6b is a voltage that is opposite in polarity to the electric charges of the toner image 1a formed on the peripheral surface of the photosensitive body 1.
While the recording paper 4 passes over the opening of the casing 6a of the transfer device 6, the discharging operation of the charge wire 6b and the electrostatic force of the toner image 1a will cause the recording paper 4 to be attracted onto the photosensitive body 1, at which time the toner image 1a is transferred onto the recording paper 4 at the same time.
The recording paper 4, after passing through the transfer device 6, is peeled off or released from the photosensitive body 1 naturally by its rigidity or forcingly by mechanical or electrical means and then guided through the fixing guide 7 to a fixing unit.
In the event where the tip end of the recording paper 4 passed through the transfer device 6 is released faster from a position upstream of a predetermined position on the photosensitive body 1, this may cause the paper 4 to be caught in a wall surface of the casing 6a of the transfer device 6 so that the paper cannot be fed forward, thus causing paper jamming.
For example, when the recording paper 4 is high in humidity content, the paper 4 becomes low in surface resistance and electric charges carried on the rear face of the paper tend to easily escape therefrom in the transfer device 6, which results in that the tip end of the paper may sometimes be released from the photosensitive body 1 at a position upstream of the predetermined position (that is, released faster than a predetermined timing).
To avoid the above defect, in the prior art transfer device, as shown in FIG. 15, there has been employed such guide means that both side walls of the casing 6a are formed respectively to have a plurality of notches 6d spaced from each other along the longitudinal direction of the side walls, such a insulating string 6e as a fishing string is alternately hooked to the respective notches 6d to be extended over the opening of the casing 6a, thereby preventing the tip end of the paper 4 from being caught in the opening.
The employment of the above guiding means has been intended to cause the string 6e of the transfer device 6 to prevent the falling of the tip end of the paper 4 into the casing 6a, thereby ensuring the positive guiding of the paper to the fixing guide 7, even when the tip end of the paper 4 is released faster than the predetermined timing from the photosensitive body 1 at a position upstream of the predetermined position.
The prior art transfer device 6 having the guiding means arranged as mentioned above, however, has had such a problem that, when the string 6e extended over the opening of the casing 6a is elongated and loosely extended due to the heat generated at the fixing unit provided adjacent to the transfer device 6, the string 6e is unhooked from some of the notches 6d of the casing 6a and thus the extended state of the string 6e over the opening is broken which results in that the falling of the tip end of the recording paper 4 into the casing 6a cannot be prevented and paper jamming takes place.
Thus, in order to reliably prevent the occurrence of paper jamming in the prior art transfer device 6, it has been necessary to monitor the extended state of the string 6e at all times.
FIG. 16 shows an electrophotographic recorder which comprises another prior art transfer device different in structure from the aforementioned prior art transfer device.
The arrangement of electrophotographic recorder of FIG. 16 is substantially the same as that of the recorder of FIG. 14 in that the recorder of FIG. 16 includes a photosensitive body 1, a developing unit 2, register rollers 3a and 3b, a recording paper 4, a register guide 5, a transfer device 6, a fixing guide 7, the parts 2, 3a, 3b, 4, 5, 6 and 7 being disposed around the photosensitive body 1. However, the recorder of FIG. 16 is different from the recorder of FIG. 14 in that a feeding rubber roller 8 having a small diameter is provided at a position downstream of the register guide 5 and close to the transfer device 6 in FIG. 16.
In the illustrated example, more specifically, the feeding rubber roller 8 is mounted to neighbor the peripheral surface of the photosensitive body 1. The roller 8 somewhat contributes to preventing the recording paper 4 from falling into the transfer device 6 in the aforementioned transfer process, but the main purpose of the roller 8 is prevent the deterioration of picture quality in the transfer process.
The deterioration of picture quality in the transfer process will first be briefly explained together with the recording operation of the recorder of FIG. 16.
The recording operation of the recorder of FIG. 16 is also carried out based on the aforementioned electrophotographic recording process.
More in detail, in the first exposure process, an electrostatic latent image corresponding to a picture signal is formed on the peripheral surface of the photosensitive body 1.
In the next developing process, the developing unit 2 is rotated in synchronism with the rotation of the photosensitive body 1 to attach developing agent to the electrostatic latent image and form a toner image 1a.
The feeding of the recording paper 4 is started from the register rollers 3a and 3b at a predetermined timing tailored to the exposure and developing processes.
Thereafter, the recording paper 4 passes through the register guide 5 and reaches the photosensitive body 1, from which the paper 4 is fed through the rubber roller 8 along the peripheral surface of the photosensitive body 1 a wound therealong and further fed forward together with the rotation of the body 1.
During the feeding of the paper 4, a transfer process is carried out in such a manner that the transfer device 6 applies corona discharge to the recording paper 4 wound around the photosensitive body 1 from the rear side of the paper to transfer onto the recording paper 4 the toner image 1a formed on the body 1.
The recording paper 4 having the toner image 1a thus transferred thereonto is peeled off from the photosensitive body 1 at its predetermined position thereon and then guided through the fixing guide 7 to a fixing process station.
In the fixing process station, the toner image 1a is positively fixed on the recording paper 4 under the influence of the heating or pressing operation of a fixing unit (not shown) and the paper having the picture image stably recorded thereon is discharged from the recorder.
With the transfer device 6 of the recorder having such an arrangement as mentioned above, when a transfer current applied from the charging wire 6b is concentrated on upstream extrusion one of both ends of the casing 6a located on the paper input (upstream) side, this tends to cause the reduction of its transfer efficiency and thus the generation of disturbance of picture quality.
To avoid this, the extrusion end of the casing 6a has been conventionally covered with an insulating material 9 to thereby impede the invasion of the transfer current into the rubber roller 8.
Further, during execution of the electrophotographic recording process, the rubber roller 8 is charged with the same polarity as the electrode of the transfer device 6 so that, before the recording paper 4 is wound round the photosensitive body 1, the roller 8 attracts the toner image 1a attached on the peripheral surface of the body 1, which results in that the picture is scattered and in an extreme example, the leak current causes the machine to stop.
As a prior art countermeasure of eliminating the above problem, a sip ring 10 has been attached to the rubber roller 8 as grounded to prevent the roller 8 from being charged.
In such a sort of transfer device 6, however, the slip ring 10 provided to suppress the rubber roller 8 from being charged, also has promoted the transfer current to be supplied from the transfer device 6 to the rubber roller 8.
As a result, in the transfer process, the transfer current directed from the transfer device 6 to the photosensitive body 1 has been decreased, whereby the transfer has become unstable and the resultant recorded picture has been partly missed.
As has been explained above, this sort of prior art transfer device has had such a problem that, since the grounded rubber roller is disposed very close to the transfer device, most of the transfer current flows into the rubber roller and the net current flowing toward the photosensitive body is decreased to a level below its necessary value, thus resulting in that the picture transfer becomes unstable with picture missing and therefore the record quality is inevitably reduced.